The invention relates to a target for a cathode disposed in an evacuable process chamber which can be flooded with a process gas in an apparatus for coating workpieces. A plurality of target tiles are disposed end to end on the cathode body and together form the target. At least one target base plate is disposed between the target tiles and the cathode body.
In sputtering processes fragile materials are often used as the target. Such materials include, for example, silicon (Si) and indium tin oxide (ITO).
Especially in the glass coating industry, sputtering apparatus with magnetron cathodes of up to 4000 mm in length are used. The material to be deposited is fastened in the form of a plane-parallel plate composed of one or more pieces onto a target base plate. During the sputtering process a predominant part of the power supplied to the cathode is given off as heat. In order to assure a sputtering process of enduring stability the target material has to be intensively cooled.
A bonded target of a fragile material is indirectly cooled. The heat produced is removed through the water-cooled target base plate. To assure good cooling the target material must have a good thermal contact with the base plate and the base plate must have a high thermal conductivity.
In the case of delicate materials, good thermal contact can be achieved only by cementing or soldering the target material to the base plate. Tight clamping (with edge cleats) is impossible on account of the fragility of the target. Copper has very good thermal conductivity and is furthermore inexpensive, and therefore target base plates are often made of copper. In high-power sputtering, temperature differences can occur between the target surface of the target assembly and the target base plate amounting to more than 100.degree. C. Since the base plate and the target consist of different materials with different coefficients of expansion, a very high tension develops at the target and base plate interface. This very often results in the deformation and in the separation of the target material from the base plate. To prevent this in the case of a large-area coating cathode the target material must be composed of several pieces and have a very good and heat-resistant adhesion to the base plate.
In practice a silicon or ITO target is composed of a plurality of tiles. These tiles are cemented or soldered onto a copper plate of the size of the cathode.
Preparing a large target (e.g., over 2 meters long) by cementing is extraordinarily time-consuming and difficult. To permit good adhesion, the cement is cured in practice at a high temperature (up to 180.degree. C.). When the finished target cools to room temperature, enormous tensions occur between the copper base plate and the target material due to the different expansion coefficients. To prevent deformation of the finished target and the destruction of the silicon tiles, certain gaps must be left between the individual silicon tiles. Upon cooling, the tiles run together and the gaps become narrower. At the high temperature, if the gaps are set too close or partially filled with cement residues, the tiles are pressed against one another with enormous force, resulting in the destruction of the target material during the sputtering process. If the gaps are set too wide, empty gaps are left between individual target parts after cooling, which leads to instability (arcing) in the sputtering process, so that the gaps between the individual tiles must be set with extraordinary accuracy, and the cement residues must be entirely removed from the gaps.
In order to prevent destruction of the entire target by deformation during transport and installation, the target base plate has to have some rigidity. This is provided by an appropriate thickness of the target base plate (6-10 mm) depending on the target length.
Soldered targets have as a rule very good thermal contact and strong adhesion of the material to the base plate. The soldering procedure is similar to cementing, although the target material in this case must be especially prepared. First one or more metal coatings are deposited on the bonding surface of the target to assure a tight join between the solder and the target material. The soldering is performed at a temperature above 100.degree. C. The problems involved in the cooling of the target are the same as those which occur in cementing.